When cyclones strike: massive flooding is a major threat to lives and property in low-lying coastal areas
As one of the worst tropical cyclones on record to affect Africa and the entire Southern Hemisphere, cyclone Idai caused disastrous damages and loss of life in Mozambique, Zimbabwe, Malawi and Madagascar. Records to date of this category 3 cyclone indicate severe flooding and strong winds (10-minute sustained wind of 195 km/h and gusts up to 280 km/h), affecting more than 2.5 million people. The current death toll from Idai is 843 people (though many people remain missing and the death toll will continue to rise). Catastrophic damage occurred in and around the port city of Beira in southern Mozambique, where Idai produced a storm surge of 4.4 m (14 ft); severe wind and flood damage occurred well away from the point of landfall.
Mangrove forests can reduce vulnerability from storm surges by obstructing the flow of water, but little use is made of this natural buffer
When cyclones strike, mangrove forests help protect coastal areas from storm surges. As the surge moves through the mangrove forest, the tree roots, trunks, and leaves obstruct the flow of water. Damage to adjacent coastal lands is attenuated mainly by reducing (i) surge height, which determines the area and depth of inundation, and (ii) water flow velocity. Although the potential utility of mangroves in disaster risk reduction is increasingly recognized by coastal managers, the use of mangrove forests is often hindered by the scarcity of location-specific information on the protective capacity of mangroves, which depends on bathymetry, characteristics of waves, suitable mangrove species, forest floor shape etc.
World Bank research findings help quantify the protection from mangroves during cyclones
Unfortunately, not all cyclone-prone countries have significant mangrove forests, due to clearing for aquaculture, tourism, industrial/urban development, and extraction of mangrove timber. A World Bank study published in Ambio quantified existing mangroves in all developing countries with previous exposure to tropical cyclones, making use of the global mangrove database of the US Geological Survey. This study also estimated potential coastal protection from mangroves for each country. The research shows that both Mozambique and Madagascar have suitable habitat for mangroves. An intact mangrove forest along the coast of Mozambique could reduce surge inundation area by 24%. However, the actual distribution of mangroves there is uneven. Of particular note, the port city of Beira and adjacent areas where Idai made landfall have little area with mangroves.
Mangroves can be used more effectively for coastal protection
To make efficient use of mangrove forests for coastal protection, site specific analysis of their protective capacity is required, as the degree of protection depends on the width of a mangrove forest, forest density, and the diameter of stems and roots of trees. Site-specific information can be used both to assess the protective capacity of existing mangroves, and the additional protection that would be provided by restoring mangrove forests in their natural habitats. To get deeper insight into the question, a World Bank study recently published in PLOS One looked at the protective capacity of mangroves for Bangladesh - one of the countries most vulnerable to tropical cyclones. The research team worked in seven coastal locations of Bangladesh and made use of hydrological models to replicate the impacts of category 4 Cyclone Sidr, which generated 4 m (13 ft) storm surge along Khulna-Barisal coast of southwest Bangladesh in 2007. The findings indicate that mangroves reduced surge height by 4–16.5 cm with mangrove strips of 50 m to 2 km width, and reduced water flow velocity by 29–92 percent with forest widths of 50 m or 100 m.
Integration of “green” infrastructure (mangroves) with traditional “gray” infrastructure (embankments) can provide cost effective protection from cyclones
A recent report released by the World Bank, the Global Facility for Disaster Reduction and Recovery (GFDRR), World Resources Institute states that integrating “green” infrastructure (mangroves and wetlands) with traditional “gray” infrastructure systems (embankments) can help deliver a “triple win,” with benefits for the economy, communities, and the environment.
For protection from cyclones, mangroves may be particularly effective in rural areas where populations are widely dispersed, and the construction long seawalls may not be economically feasible. In contrast, for densely populated coastal regions, mangroves alone will not fully protect populations, assets, and activities at risk since the reduction of surge height from mangroves may be modest. However, even a modest reduction in surge height provided by healthy mangroves in the foreshore of embankments will allow embankment heights to be lower, thereby reducing the cost of construction considerably. In addition, the significant reduction in water flow velocity from planting mangroves will protect embankments from damage and thus reduce infrastructure maintenance costs. When designing embankments, the benefits and costs of mangrove planting should be considered early on.